It is well known for electroless metal plating on an insulating substrate such as plastics to previously deposit an initiator for electroless metal plating such as palladium (Pd) on the surface of the substrate and then to immerse the thus treated substrate in an electroless metal plating solution.
According to said method, a printed circuit is produced by selective deposition of electroless metal on only the circuit area. For this purpose, said initiator such as Pd is selectively deposited on only the circuit area or Pd is deposited on the whole surface and then areas which are not to be subjected to electroless metal plating are masked with resin and the like. Since the selective deposition of the initiator as in the former method is difficult, the latter method has been proposed. For example, see U.S. Pat. No. 3,666,549. However, even by the latter method, it is impossible to prevent deposition of electroless metal on the resin which masks the undesired areas. Even such slight deposition of electroless metal causes short pass in printed circuits which require high density and accuracy. Therefore, such deposition of electroless metal on the areas other than the circuit areas must be prevented by all means.
It is not clear why electroless metal is deposited on the masking resin, but it is supposed that a part of the initiator such as Pd which is deposited on the circuit areas is shed and deposited on the resin surface. It is also supposed that the fine surface imperfections present on the masking resin have catalytic action for deposition of the electroless metal. The surface imperfections in this case mean areas on which electroless metal is deposited without deposition of initiator. It has been proposed to prevent deposition of electroless metal on the areas other than the circuit areas by applying materials capable of neutralizing the surface imperfections. For example, see U.S. Pat. No. 3,443,988. However, since printed circuits which have a plated film of large thickness require a long plating time, deposition of metal on unwanted areas cannot be prevented only by such method.